The present invention generally relates to image transfer technology and, more particularly, to an apparatus for charging image transfer surfaces of image transfer devices during the printing process, and an image transfer device having the apparatus.
As used herein, the term “image transfer device” generally refers to all types of devices used for creating and/or transferring an image in an electrophotographic process, including laser printers, copiers, facsimiles, and the like. As used herein, the term “electrophotographic process” includes both dry and liquid electrophotographic (LEP) processes.
In an electrophotographic image transfer device, the surface of a photoconducting material (i.e., a photoreceptor) is charged to a substantially uniform potential so as to sensitize the surface. An electrostatic latent image is created on the surface of the charged photoconducting material by selectively exposing areas of the photoconductor surface to a light image of the original document being reproduced. A difference in electrostatic charge density is created between the areas on the photoconductor surface exposed and unexposed to light. For example, in a liquid electrophotographic process, the photoconductor surface is initially charged to approximately −1000 Volts, with the exposed photoconductor surface discharged to approximately −50 Volts. Alternatively, the photoconductor surface can be initially charged to 1000 Volts, with the exposed surface discharged to approximately 50 Volts.
The electrostatic latent image on the photoconductor surface is developed into a visible image using electrostatic toners or pigments. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and toner. The photoconductor surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or other medium is passed close to the photoconductor surface, which may be in the form of a rotating drum or a continuous belt, transferring the toner from the photoconductor surface onto the paper in the pattern of the image developed on the photoconductor surface. The transfer of the toner may be an electrostatic transfer, as when the sheet has an electric charge opposite that of the toner, or may be a heat transfer, as when a heated transfer roller is used, or a combination of electrostatic and heat transfer. In some printer embodiments, the toner may first be transferred from the photoconductor surface to an intermediate transfer medium, and then from the intermediate transfer medium to a sheet of paper.
Charging of the photoconductor surface may be accomplished by any of several types of charging devices, such as a corotron (a corona wire having a DC voltage and an electrostatic shield), a dicorotron (a glass covered corona wire with AC voltage, and electrostatic shield with DC voltage, and an insulating housing), a scorotron (a corotron with an added biased conducting grid), a discorotron (a dicorotron with an added biased conducting strip), a pin scorotron (a corona pin array housing a high voltage and a biased conducting grid), or a charge roller. In general, charge rollers are used with image transfer devices having slower throughput, while corotrons, scorotons, and the like are used with image transfer devices having faster throughput.
Charge rollers having a variety of designs are known in the art. The elastomeric portion of a charge roller typically assumes one of two configurations. One charge roller configuration is a single-layer elastomer with a moderately conductive material, such as an ionic conduction agent, mixed into the elastomer. The single-layer charge roller may optionally have a very thin (on the order of a few microns) layer of insulating material on its exterior surface. The other charge roller configuration is a double-layer construction having a thicker (on the order of a hundred microns and greater) insulating outer sleeve and an inner elastomeric region loaded with a network of highly conductive material, such as carbon black. The double-layer charge roller configuration generally charges the photoconductor surface less uniformly due to the difficulty in obtaining a constant thickness and resistivity for the outer insulating sleeve.
The ability to use-charge rollers in high-speed high quality image transfer devices is limited by several factors. In particular, currently available charge rollers are unable to provide the required charging voltages at the necessary current frequencies while having a satisfactory lifespan. There is a need for a charge roller capable of use in high-speed high quality image transfer devices.